Many types of multi-speed transmissions are available for motor vehicles. In such transmissions, shifts from one operating mode, range, or ratio to another operating mode, range or ratio may be controlled electronically by a transmission control system that includes computer circuitry, programming logic, and fluid hydraulics installed in the vehicle. In general, an assembly of electro-hydraulic actuators (such as solenoids), hydraulic valves and fluid passages makes up an electro-hydraulic portion of the transmission control system. Electrical controls selectively actuate hydraulic valves, which control the pressure and direction of hydraulic fluid flow applied to torque transmitting mechanisms (such as clutches, brakes and the like) and other components of the transmission. The clutches or other torque transmitting mechanisms may be engaged and disengaged upon command to attain different operating modes, ranges or ratios for the vehicle.
Some of the design considerations for transmission control systems include shift quality, fuel economy, reliability and durability over a wide range of potential operating conditions. The range of operating conditions may include changes in the weight of the vehicle load, operating temperature, weather conditions, geographic conditions (such as altitude or humidity), and driver preferences. The configurations of component parts of the transmission and/or the transmission control system may vary. Variations may also occur in the reliability, durability and/or manufacturing tolerances of component parts of the transmission and/or control system.
If a failure or malfunction occurs in the transmission during vehicle operation, it may be desirable or necessary for the transmission control system to transition the vehicle to a safer or more desirable state until the failure can be addressed. In other instances, it may be desirable or necessary for the transmission control system to prevent the transmission from causing the vehicle to enter an operating mode that is known or believed to be undesirable or unsafe.